In packing technology, packages of a non-returnable character which are manufactured from a material comprising a base layer of paper or cardboard with outer and inner coatings of thermoplastics have been used for a long time. The material in these so called non-returnable packages is often also provided with further layers of other material, e.g. Al-foil or plastic coatings other than those mentioned here.
The composition of the packing material is selected to create the best possible protection for the product which is to be packed. At the same time, the composition is selected to give the required mechanical strength and durability to enable it to withstand such outer external stresses as the package is subjected to in normal handling. To achieve the necessary mechanical rigidity, which on the one hand gives mechanical protection to the product and on the other hand makes it possible for the package to be dimensionally rigid enough to allow it to be handled and manually gripped without difficulty, the material in these packages is frequently provided with a relatively thick base layer of paper or cardboard. Such a material, however, is not liquid or gas light and the rigidity of the material is quickly lost when it is subjected to moisture. To impart the required liquid-tightness to the material the base layer is provided therefor, frequently on both sides, with a coating of plastic material, and if the plastic material is thermoplastics the coating may also be used for sealing the plastic coatings to one another by so-called heat-sealing. In this manner packages can be sealed and made lastingly permanent in their intended shape by heat-sealing together thermoplastic-coated, overlapping material panels in liquid-tight and mechanically durable sealing joints.
Non-returnable packages of the type referred to here are manufactured in most cases with the help of packing machines which, either from a web or from prefabricated blanks of a packing material, form, fill and seal finished packages at a high rate of production. Packages are manufactured, e.g. from a web by joining together the longitudinal edges of the web in an overlap joint so as to form a tube which is substantially filled with the actual contents, and through repeated flattening and sealing of the tube, at right angles to the longitudinal axis of tube, is divided into closed packages. Finally, the packaging units are severed from one another by means of cuts in the transverse sealing zones and are given the desired geometrical shape, usually a parallellepiped, by further folding and sealing.
During the manufacture of packages in the manner described above the laminated material is subjected to stresses which become particularly large during folding of the material, During folding, and as a result of the relatively great material thickness of the base layer, the one plastic coating is subjected to a strong stretching force while at the same time, the other plastic coating is compressed to a corresponding degree along the folding line. Due to the great extensibility of the plastic coatings, the folding and resultant stretching of the plastic material only rarely leads to breaks or other damages causing leakage in the plastic coating. However, the problem is aggravated if the material also comprises an layer which, compared with the plastic coating, is not as stretchable and consequently tends to crack when the material is folded.
Even if a single 180.degree. folding of the material normally does not have any serious consequences, considerable difficulties arise when the material is to be folded along two crossing crease lines. This is often the case in external sealing areas always occurring on this type of packages, irrespectively of whether they are manufactured from a web or from prefabricated blanks. The sealings generally are carried out by heating and melting the plastic coating facing towards the inside of the package along the edge zones which are to be sealed to one another. Thereafter, the heated plastic coatings are pressed against one another to form a sealing fin held together through surface fusion on the outside of the package. Such a fin comprises double material layers, and to ensure that it does not form an obstacle, the fin frequently is folded down to lie flat against the outside of the package, which means that one of the material layers of the sealing fin is folded over a 180.degree., and that the package wall in the region of the folded-down fin comprises three material layers, that is to say, has a threefold material thickness. Such a sealing fin often runs along one or more side faces of the package, and since these side faces in the forming of, for example, parallellepipedic packages are subjected to a 180.degree. folding along a crease line at right angles to the sealing fin, the material thickness in certain regions of the package will be six times the laminate thickness. At this In the area of the 180.degree. folding that is transverse to the sealing region the material layers located outermost will be subjected to very strong tensile stresses with accompanying extensions and increased risks of crack formation in the material. These tensile stresses frequently are so great that cracks occur not only in the Al-foil included in the material, but also in the thermoplastic coatings. Cracks in the thermoplastic coating can result in leakage of the packed contents which can readily be absorbed by, and impair the rigidity of, the base layer of the material.